The present invention is directed to a composition and method of selectively electroplating a tin or tin alloy on a composite substrate having metallic portions and ceramic portions without loss of adhesion between the metal and ceramic portions. More specifically, the present invention is directed to a composition and method of selectively electroplating a tin or tin alloy on a composite substrate having metallic portions and ceramic portions without loss of adhesion between the metal and ceramic portions where the composition has components that do not compromise adhesion between the metallic and ceramic interface.
The electronic components industry is continually searching for new composite materials for use in electronic components. Such new materials are being developed in accordance with the need for miniaturized parts, which are capable of performing various operations.
Many of these composite materials include metallic portions, which alone may be easily electroplated utilizing state of the art electroplating baths. When such electroplatable portions are combined in a composite material in combination with a non-conductive, non-electroplatable portion such as a ceramic, selectively electroplating the metallic portions without affecting the ceramic portions becomes very difficult. For example, when tin/lead fluoroborate electroplating baths are utilized to achieve selective plating, the fluoroboric acid attacks the ceramic portions of the composite substrate. This attack may be in the form of solubilizing (i.e., etching or dissolving the ceramic material) or cracking. The undesired etching may result in the tin/lead alloy depositing on the ceramic, thus causing a short circuit of the metallic (electroplatable) portions.
Many tin/lead electrolytes, which are not based on fluoroborates, also are not suitable for electroplating composite materials. While non-fluoroborate baths do not cause cracking of the ceramic portions, they deposit tin/lead alloys upon such portions, thus causing short-circuiting of the metallic portions.
One type of electronic component, which utilizes a composite structure, is a dual inline package (DIP). Such components may be prepared by adhereing a die or chip to a surface and connecting the circuitry to a plurality of pins. The die is then encapsulated with ceramic pieces that are adhered together with a soft lead oxide glass containing 50% lead oxide that melts at relatively low temperatures. The soft glass binder used to encapsulate the circuit oozes around these pieces and hardens during the fabrication to become part of the package. The structure is composed of metallic pins, the encapsulating ceramic, and the hardened soft lead oxide glass encapsulation. In order to continue processing of these packages, the metallic portion (usually an iron-nickel alloy) is electroplated to facilitate soldering connection to the pins.
The common practice in the industry was to plate these DIPs with pure tin from a solution containing stannous sulfate and sulfuric acid, since the soft lead oxide glass portions do not accept a tin deposit from such solutions, nor do such electroplating solutions adversely affect the soft lead glass portions. Pure tin electrodeposits, however, have been known to produce whiskers, which grow out from the surface of various directions. These whiskers have the appearance of very fine hairs of tin metal, which can bridge adjacent metal parts and cause a short circuit.
Attempts have been made to electroplate DIPs with a tin/lead alloy containing more than 5% lead or more, since such tin/lead alloys do not have a tendency to form whiskers. However, such electroplating baths may not be used for many composite substrates such as those having a softlead glass binder. Such binders become electroplated, thus causing short-circuiting. Additionally they are soluble or are otherwise adversely affected by such plating baths.
The problems with selectively plating DIPs was addressed and solved as described in U.S. Pat. No. 4,640,746 to Nobel et al. The patent discloses a tin or tin/lead alloy electroplating bath containing a wide variety of complexing agents to maintain tin and lead materials in solution, an imidazoline compound, an alkylene oxide as well as grain refiners or brighteners and pH adjusters such as sodium or potassium hydroxide and ammonia or an ammonium salt. Preferred complexing agents are sodium gluconate, potassium pyrophosphate, potassium sodium tartrate, ammonium citrate, hydroxy ethylidene diphosphonic acid, nitrilo tri-methyl phosphonic acid, and hydroxy acetic acid or one or more of their salts. The pH at which the bath is employed is 1.5 to 5.5. The electroplating bath selectively plated the metallic portions of the DIPs without causing bridging or short-circuiting between adjacent spaced metallic portions. However, portions of the lead oxide glass ceramic were still solubilized. This effect was within acceptable limits of the industry for DIPs.
However, acceptable limits vary depending on the substrate to be electroplated and its function in an electrical device. Tolerable dissolution of ceramic material for DIPs may be unsatisfactory for many electronic components such as for embedded passives, i.e. capacitors or resistors where optimum adhesion between a metal and a ceramic material is critical to electronic performance. Further, as the electronics industry continues to miniaturize electronic devices and components the industry demands further reduction of defects. While the tin and tin/lead alloy electroplating baths of the Nobel patent were affective in the manufacture of DIPs, the baths were found to compromise adhesion between metal pastes and ceramics at their interface.
Another type of electrical component that utilizes a composite structure is the multilayer ceramic chip capacitor (MLCC). The construction of a MLCC comprises a ceramic block with embedded, inter-digitated electrodes. The electrode surfaces are exposed at the two opposite ends of the ceramic block, where contact is made by the application and high temperature firing of a conductive paste. This forms what are referred to as the terminations. The inner layer electrodes in the ceramic block can vary from a few to a few hundred.
There are two primary types of MLCC's commonly referred to as precious metal electrode (PME) MLCC's and base metal electrode (BME) MLCC's. In PME MLCC's the inner electrodes are either pure palladium or mixtures of palladium and silver. The high temperature fired conductive paste, which forms the terminations is silver based. Sometimes it also contains palladium. In the case of BME MLCC's, the inner electrodes are nickel or mixtures of nickel and copper, and the conductive paste is copper based. The final step in making both BME and PME MLCCs' is to tin or tin-lead plate the terminations to provide a solderable surface for assembly of chip capacitors to printed circuit boards.
Because of the relative price of palladium versus nickel and copper, there is a desire of MLCC manufacturers to utilize BME methodologies. Many MLCC suppliers are in full production with BME products.
Conductive pastes, as described above, at the terminations of base metal electrodes for multilayer ceramic chip capacitors often de-adhere from the BME in tin plating electrolytes. The pH of many tin electrolytes is kept above 3 to minimize attack of the ceramic chip. The terminations on the BMEs often “lift” or de-adhere after they are exposed to the electrolyte. The “lifting” has been associated with certain organic complexing agents, such as organic acids, added to the tin electrolyte in order to keep stannous tin in solution at pH 2 and above. Accordingly, there is a need for an improved tin and tin/alloy electroplating bath that does not compromise adhesion between metal pastes and ceramic materials.